This invention relates to the production of electrically conductive polyaniline and is particularly concerned with a process for producing conductive polyaniline having controlled molecular weight and good solubility in organic solvants.
Electrically conductive polyaniline is generally synthesized via the oxidative polymerization of aniline under aqueous acidic conditions. More specifically, in the oxidative polymerization of aniline, a strong oxidant such as ammonium persulfate is added to an aqueous HCl solution containing aniline. The strong oxidant is needed to oxidize aniline. The aniline then undergoes oxidative polymerization to form conductive polyaniline. This material can be de-protonated, if desired, in an ammonium hydroxide solution to form the non-conductive free base polyaniline.
Aniline is much more difficult to oxidize than the polyaniline oligomers. Consequently, the slow or rate-determining step of the reaction is the oxidation of aniline. This is very slow. Once an aniline molecule is oxidized, other aniline moieties attach and polymerize onto it. This polymerization reaction is highly exothermic. Because aniline quickly polymerizes off of the first aniline molecules oxidized, the molecular weight distribution of the polymer is difficult to control and can range widely. The polyaniline product of such reaction generally contains a large percentage of polymer of very high molecular weight, e.g. over 2,000,000. However, this same product also contains some material having a relatively low molecular weight, e.g. of the order of 2,000. Unfortunately, only the low molecular weight material is soluble in organic solvents. The solubility of high molecular weight conductive polyaniline in organic solvants has been increased by attaching specific derivatizing agents such an anhydrides to the polymer backbone. This has been described in U.S. Pat. No. 4,851,487 to S. I. Yaniger and Randy E. Cameron, and assigned to the same assignee as the present application. U.S. Pat. No. 4,855,361 to S. I. Yaniger and Randy E. Cameron, and assigned to the same assignee as the present application, describes the blending of such soluble conductive polymers with polyimide resins.
The utility of conductive polymers such as conductive polyaniline is optimized when the material is highly soluble in organic solvants. This solubility allows for easy blending of the polymer with other resins such as polyimides as described in above U.S. Pat. No. 4,855,361. It is well known that the solubility of a polymer is increased when its molecular weight is decreased. Consequently, the optimum conductive polyaniline for blending should have as low a molecular weight as possible while maintaining high electrical conductivity and good thermal stability. It has been found that the conductive form of polyaniline is very conductive when it is only 8 aniline units long. On the other hand, high molecular weight polyaniline is necessary for application such as for making conductive fibers. For such applications contamination by low molecular weight polyaniline decreases the strength of the fibers. Consequently each application requires a different molecular weight polyaniline. However, an easy method for controlling the molecular weight of polyaniline has heretofore not been known.
An object of the present invention is the provision of procedure for the oxidative polymerization of aniline to conductive polyaniline having controlled molecular weight.
Another object is to provide relatively simple procedure for oxidative polymerization of aniline in high yield to conductive polyaniline while controlling the molecular weight of the polyaniline to obtain relatively low molecular weight conductive polyaniline having good solubility in organic solvants, or relatively high molecular weight conductive polyaniline having good mechanical properties, e.g. for making fibers.
Still another object is to provide procedure for synthesizing low molecular weight conductive polyaniline having good solubility in organic solvants, and whose solubility can be further optimized, if desired, by deprotonating to the polyaniline emeraldine free base form and derivatizing the polyaniline with derivatizing agents such as an anhydride.
Yet another object is to provide novel procedure of the above type for oxidative polymerization of aniline to conductive aniline employing weak as well as strong oxidants and obtaining, in high yield, a product of good purity.